Resonant cavity device



June 2, 1942.

E. G. LINDER RESONANT CAVITY DEVICE Filed Aug. 3l, 1939 Patented June 2, 1942 2,284,751 l m-:soNAN'r cAvrrY Device .Ernest G. Linder, ihiladelphia, Pa., assignor to Radio Corporation o! America, a corporation of Delaware Application August 31, 1939, Serial No. 292,798

-(Cl. Z50-27) 11 Claims.

This invention relates to resonant cavity devices and more particularly to resonant cavity devices in which electrons passing through the device are decelerated, accelerated, or modulated by appropriate interaction with the iields of the standing waves.

Resonant cavity devices are known to those skilled in the art. In such devices, standing waves are established by an external source or by f self-oscillation. The electric eld of the waves is generally parallel to the axis. According to the present invention, electrons are passed through the axis of a cylindrical resonant cavity. Ordinarily, electrons passing through the cavity along its axis would be alternately accelerated and decelerated, but aseries'of suitably spaced and suitably proportioned shields make the field effectve only on alternate half cycles. Thus, either acceleration, deceleration or modulation of the electrons is greatly augmented.

One of the objects of the invention is to provide means for accelerating, decelerating, or modulating electrons. Another object is to provide a resonant cavity device by means of which oscillation, amplification, or demodulation may be obtained. A further object is to pass electrons through an alternating eld and, by means o shields, prevent the field from acting upon the electrons during alternate half cycles whereby acceleration or deceleration may be substantially increased.

The invention will be described by vreferring to the accompanying drawing in which Figure l is a schematic diagram of a resonant cavity oscillator; Figure 2 is a schematic diagram of a resonant cavity amplifier; Figure 3 is a schematic diagram of a resonant cavity demodulator; and Figures 4 and 4a are schematic diagrams of the elevational and sectional views of a magnetic iield type of resonant cavity device. Similar reierence numerals will be applied to similar elements in the several figures.

Referring to Fig. 1, within an evacuated envelope I are mounted a member-forming cylindrical resonant cavity`5?, an electron source 5, a control electrode 1, an anode 9 and an output pickup loop I I. It should be understood that the resonant cavity may be a portion of the envelope rather than an entirely separatev member. In either event, the ends of the cavity each include an aperture I3 through which electrons from the source 5 are projected toward the anode 9. A number of spaced cylindrical shields I5 are positioned along the axis of the cavity. The shields are so dimensioned and positioned that they are eiective only on alternate haii cycles. In the case of an oscillator, since the electrons are decelerated, the shields decrease in length and the spaces between the shields decrease as the anode is approached. Furthermore, the diameter of the shields should be below the critical dimension which would permit the waves from propagating within the shielded region.

A The operation of the oscillator is as follows: Electrons are projected through the cavity and a transient effect establishes waves within the cavity which is resonant to the applied frequency. The electric iield of the waves is represented by the dash lines IB. Some of the electrons entering the cavity are so phased that they give up energy to sustain the Awaves and thus these electrons are decelerated. When the elec tric field changes sign, some oi these electrons are passing through the rst of the shields. When the electric eld is of its original sign, some of the decelerated electrons are emerging from the first shield and again give up energy. After giving up energy the second time, the electrons again pass within the second shield so that they do not abstract energy from the electric field. The method or steps continue until some of the original electrons pass through the final shield and reach the anode.

Although the device may be used without attempting to regulate the phase of the electrons passing into the cavity, greater eiiiciency can be obtained by coupling the output ll through a coupling loop Il, and phase adjusting means i9 to the control electrode l. The control electrode is thus biased by currents derived from the output and is thus able to control the phase of the electrons entering the cavity.

The efficiency without shields is indicated by the formula n -LEZI in -cll wiwi/0z s V0 where n=cficiency; e=electron charge; m=

electron mass; E=eld strength along axis; l= resonator length; `w=angular Vfrequency; V= electron velocity at entrance. This formula shows that the operation is a periodic function of the length of the cavity and that an electron passing through the cavity (in the absence of shields) alternately gains and loses energy. The shields may be so arranged that, as in the case of an oscillator or' amplifier, the majority of electrons only lose energy. If the device is to be used to increase the energy of the electrons, the

shield arrangement should besuch *that the shield and gap lengths increase as the anode is approached.

Since the ampliiier construction and operation is substantially similar to the oscillator, it is not necessary to repeat the description. Reference is made to Fig. 2, which shows an amplifier arranged according to the invention. It will be noted that the amplifier has both input and output connections and that the balance of the device is the same as the oscillator.

When the device is used for demodulation, the shield lengths 2| and gaps 23 are made oi similar lengths as shown in Fig. 3. Furthermore, the anode 9 is connected through the primary 25 of an output transformer 21 to the anode battery. The telephones 29. or other signal indicator or amplifier, are connected to the secondary of the transformer. The demodulation of modulated radio frequency signals, applied to the input circuit, is by velocity modulation of the electron stream.

Although the above-described devices represent the preferred embodiments of the invention, it is not limited thereto but may be applied to a magnetron type of device as shown in Figs. 4 and 4a. 'I'his type utilizes the so-called H0 type of Wave in which the electric lines of force are circles concentrically disposed with respect to the axis of the cylinder 3|. The cylinder 3l has a reentrant portion 33 which includes slots 35 and which serves as an anode. Within the reentrant portion is arranged a cathode 31. The cathode and anode, together with a suitable magnetic field partially indicated by the pole pieces N S, form a magnetron.

The operation is substantially as follows: Electrons emitted by the cathode under the iniluence of the magnetic ield and electric eld spiral around the cathode; The electrons have a component of motion 38 in the direction of the field 4| of the standing wave. The slotsare so arranged that the electrons are only exposed to the eld during a portion of the cycle. 'I'he electron velocity is adjusted so that certain electrons deliver energy to sustain'the standing waves.

Thus the invention has been described as an y improved resonant cavity device in which the iield of the standing waves within the cavity mutually reacts with electrons which are projected through the cavity. The electrons pro, jected through the cavity are alternately shielded and exposed to the field so that the electrons may be phased to obtain an efllecient transfer of energy. The device may be used as an oscillator, amplifier or demodulator.

I claim as my invention:

1. A resonant cavity device including in combination a member including a resonant cavity within which a field of standing waves is established, means for projecting lelectrons within said cavity, and means located within said cavity for shielding said electrons so that alternate half cycles of the iield of standing waves within said cavity react upon said electrons.

2. A resonant cavity device including in combination a member forming a cylindrical resonant cavity within which fields of standing `waves are established, means for projecting electrons within said cavity along its axis, and a plurality of shields located within said cavity and so spaced that said electrons are shielded from the fields established by alternate half cycles of standing waves within said cavity.

3. In a. device of the character of claim 1, means for adjusting the phase at which yelectrons are projected within said cavity with respect to the phase of said standing waves.`

4. A resonant cavity device including in combination a member forming a resonant cavity within which elds of standing waves are established, means for projecting electrons within said cavity, and a plurality of/ shields of different lengths located within said cavity and so spaced and arranged that said electrons pass therethrough and are shielded from the iields established by alternate half cycles of standing Waves within said cavity.

5. In a device of the character of claim 4, a control electrode for determining the phase at which electrons may enter said cavity with respect to the phase of said standing waves.

6. In a device of the character of claim 4, means -for deriving energy from the standing waves within said cavity.

'7. In a device of the character of claim 4, means for impressing resonating waves on said device, and means for deriving amplified vcurrents from said resonant cavity.

8. In a device of the character of claim 1, means for impressing modulated waves on said cavity, and means for demodulating said waves.

9. A device of the character of claim l, in which the shielding means are so dimensioned that the waves within said resonant cavity are highly damped Within the shielded portion.

1Q. A resonant cavity oscillation generator including in combination a member including a resonant cavity within which a field of standing waves is established, means for projecting electrons Within said cavity, means located within said cavity forv shielding said electrons so that alternating half cycles of the field of said standing Waves react upon said electrons and subtract energy therefrom to maintain said standing waves, and means for deriving oscillatory energy from said standing waves.

1l. A resonant cavity device including in combination a member including a resonant cavity within which a field of standing waves is established, means for projecting electrons within said cavity, and means located Within said cavity for shielding said electrons so that half cycles of the same sign of the iield of standing waves within said cavity react upon said electrons.

ERNEST G. LINDER. 

